The Law of the Third Girl

It is a law that is immutable, dating back to the first period of time that there were more than three girls in the Garden of Eden or perhaps before the flood.

And yet this law is unwritten, a silent echo of 'the way things are supposed to be' that resonates with girls and women of all ages.
The application of this law is erratic, because while all females are afflicted with it at a young age, it does not rear it's ugly head all the time.
This law is really more of an observation I have made watching my sister grow up, watching my daughters grow up and in observing the relationships of my wife and her friends as well.

It doesn't make sense, it doesn't mean that any particular girl is bad, or wrong in any sense, to the contrary, the relationships seem to only show strain when the LAW is present and active. This is how it works.
There are three girls (women).

Girls A, B and C. Girl A and Girl B can interact nicely and quietly for long periods of time, until Girl C is introduced. Now the interesting dynamic occurs of which two girls will pair up against the third. Girl A and C can get along fine together.
Girl B and C can get along fine together.

Often times, it is the girl who is the latecomer who is regarded as the 'other' and the two girls will in essence team up to make sure the third girl feels as if she were an outsider. The Law creates what I have dubbed the Molehill Mountain effect.

Where any nonsensical disagreement between two of the girls (what she chose to wear, the song she is listening to, the movie she likes, etc) turns into a raging battle of the wills in which the third girl is forced to choose a side.
Once the third girl is out of the equation though, the argument dissipates rapidly, and two of the three decide to get along again.
I see this in my four daughters, Girl L, Girl E, Girl R and Girl H.

At any given point in time any two of them get along like sisters should.
And this combination changes as frequently as my little fashion plate model child changes clothing. Girl L and H are getting along fine.

Girls E and R are playing or studying together.
But when the youngest, H, decides to leave the safety of playing with her big sister, L, she interjects into the dynamic between E and R. Sometimes they team up against her, other times, R and H will team up against E. Then the fights really begin. But this affects females my wifes' age too.
(25 if asked) (43 if you need verification, don't tell her I said that.
) She gets along perfectly well with just about anyone, but once the Law is invoked there are those who cannot understand how she can speak with Woman X or Mrs.

Y.
Once Mrs.
B sees other friends of hers speaking to someone outside of her 'circle' the Molehill Mountain Effect creates imagined wrongs committed by others on her. But her relationship with Woman X or Mrs.
Y remains intact as long as it's not all three of them together, or as long as the combination of the three women does not include someone she considers to be in another 'circle'. I don't understand this law, I have merely observed it at work for the last 40 years and rarely does it fail.
I don't claim to have the answers, just a few strategies.

What to Do? Short of choosing our two favorites and sending the two others to boarding school, we just have to let them sort it out amongst themselves. I observe, take note and chalk it up to this unspoken Law of the Third Girl.

Strategies for Daughters - Each daughter is different and cherishes something different. One insists on getting enough study time, another really likes her television and computer viewing, another really likes her clothes, and the youngest is the only one of four with her own bank account, she likes money.

Getting it, counting it and loaning it out at 40% interest to her sisters or parents. If the law kicks in and there are three that are not getting along we simply remove what it is that they love most, until such time they can get along and show love to each other.
(secretly this is kind of fun) Strategies for Wives - It won't matter too much because seriously, when was the last time your wife listened to you? Anyway, try to make sure that the law doesn't kick in by making sure that she stays within the limit of a one friend at a time and keeps away from the insane 'third' girls. When she is upset and in tears you just have to be there to make sure that you comfort her with the caution that you have to find out which one of the friends is the 'third girl' so you don't start attacking the wrong one.
Keep the questions generic until you know.
"what did she do this time?" "how can you even stand her?" See? no reference to who the 'her' is that you are asking about, wait until you know for sure before continuing.
Perhaps this is a little tongue in cheek, but the next time you see the symptoms, you will know that you are dealing with this little discussed and yet very powerful law that can't be explained.